Muscle wasting is a debilitating complication of catabolic conditions including chronic kidney disease (CKD), diabetes, cancer or serious infections. Unfortunately, there are few reliable strategies that block the loss of muscle protein initiated by these conditions. Previously, it was found that myostatin, a negative regulator of muscle growth, is increased in muscles of mice with CKD and when myostatin is inhibited with a “humanized” myostatin peptibody, CKD-induced muscle wasting was blocked (Zhang et al., 2011a). A similar conclusion was reached in studies of mouse models of cancer cachexia (Zhou et al., 2010). In the mice with CKD, inhibition of myostatin reduced circulating levels of IL-6 and TNFα suggesting a link between inflammation and muscle wasting as reported in clinical studies (Carrero et al., 2008; Hung et al., 2011). The evidence that inflammation stimulates muscle wasting includes reports that infusion of TNFα, IL-6, IL-1β or IFN-γ into rodents results in muscle wasting while neutralization of cytokines using genetic or pharmacological approaches attenuates muscle wasting (Cheung et al., 2010). For example, rodents were treated with a constant infusion of angiotensin II (AngII) and found there was muscle wasting plus increased circulating levels of IL-6 and increased expression of SOCS3 with suppressed insulin/IGF-1 signaling; knockout IL-6 from mice suppressed Ang II induced muscle wasting (Zhang et al., 2009; Rui et al., 2004; Rui et al., 2002).
Responses to IL-6 or INFγ involve stimulation of intracellular signaling pathways including activation of Janus protein tyrosine kinases (JAKs). Subsequently, JAKs mediate tyrosine phosphorylation of Signal Transducer and Activator of Transcription (STAT) factors followed by their dimerization, nuclear translocation and activation of target genes (Horvath, 2004). Among the seven members of the Stat family, Stat3 is the major member that is activated by the IL-6 family of cytokines (Hirano et al., 1997; Kishimoto et al., 1994). Recently, Bonetto et al reported the results of a microarray analysis of muscles from mice with cancer-induced cachexia. Components of 20 signaling pathways were upregulated, including IL-6, Stat3, JAK-STAT, SOCS3, complement and coagulation pathways. Therefore, the Stat3 pathway could be linked to loss of muscle mass but the pathway from Stat3 to muscle wasting is unknown.
A potential target of activated Stat3 is C/EBPS. The C/EBP transcription factors (C/EBP-α, -β, -γ, -δ, -ω, and -ξ) are expressed in several tissues and act to regulate inflammatory and metabolic processes (Ramji and Foka, 2002). C/EBP-β or -δ can stimulate intracellular signaling in hepatocytes or inflammatory cells (Poli, 1998; Akira et al., 1990; Alonzi et al., 1997) and in mice responding to an excess of glucocorticoids, the expression and binding activity of C/EBP-β and -δ in muscle are increased (Penner et al., 2002; Yang et al., 2005).
One embodiment that includes C/EBPδ involves increased myostatin expression because the myostatin promoter contains recognition sites for glucocorticoid receptors, forkhead transcription factors as well as members of the C/EBP family of transcription factors (Ma et al., 2003; Allen and Unterman, 2007). In the present disclosure, an intracellular signaling pathway in cultured myotubes is identified that bridges the gaps between p-Stat3 and myostatin and loss of muscle mass. To examine if the pathway was operative in vivo, it was studied how two catabolic conditions, CKD or acute, streptozotocin-induced diabetes affect muscle metabolism in a muscle-specific Stat3 knockout (KO) mouse. It was also tested whether a small molecule inhibitor of Stat3 phosphorylation would correct muscle wasting. Interruption of Stat3 improved muscle metabolism and strength in mice with CKD and evidence was gathered for the pathway in muscle biopsies from patients with CKD.
The present disclosure satisfies a need in the art to provide novel compounds and methods for treating and/or preventing muscle wasting or cachexia in individuals.